The Core Working Principle of VFD

The operation of a frequency converter can be simplified into three key stages:

1.Rectification

• Input: Power-frequency alternating current (AC, e.g., 50Hz/60Hz).
• Process: A rectifier bridge (a set of diodes) converts the alternating current into direct current (DC) with a fixed direction but fluctuating magnitude.
• Output: Direct current regulated by Pulse Width Modulation (PWM).

2. Filtering

• Process: The fluctuating PWM-regulated DC passes through large-capacity capacitors on the DC bus for filtering and energy storage, making it smooth and stable.
• Output: Smooth and stable direct current.

3.Inversion (the core step)

• Input: Smooth direct current.
• Process: This is the “brain” and “limbs” of the frequency converter. Based on user settings (e.g., target speed), the control circuit (CPU) uses PWM (Pulse Width Modulation) technology to control the power switching devices (IGBTs) to turn on and off rapidly and regularly at an extremely high frequency.
• Effect: This series of precisely controlled switching actions “cuts” the direct current into a series of voltage pulses with variable widths. From a macroscopic perspective and that of the motor load, these pulses are equivalent to a three-phase alternating current with adjustable frequency and voltage.
• Output: Alternating current with variable voltage and variable frequency, which drives the motor to operate at the required speed.

Why can a frequency converter control motor speed?

Take the most common AC induction motor as an example. Its speed formula is:

Speed=(120×Frequency)/ Number of Motor Pole Pairs

① The number of motor pole pairs is an inherent physical structure of the motor and remains constant.

② Therefore, the most effective way to change the motor speed is to change the frequency of the power supply.

③ Reason for voltage regulation simultaneously: The magnetic flux inside the motor needs to be kept constant to achieve efficient operation. According to motor principles, magnetic flux is proportional to the ratio of “voltage/frequency”. If only the frequency is reduced without lowering the voltage, magnetic saturation will occur, causing the motor to overheat and burn out. Thus, while reducing the frequency, the frequency converter must decrease the output voltage proportionally. This is the origin of VVVF (Variable Voltage Variable Frequency).

What are the advantages, disadvantages and main application scenarios of V/F control and vector control for frequency converters?

1.V/F Control (Voltage/Frequency Ratio Control)

• It is the simplest and most commonly used mode, which ensures a constant ratio of output voltage to frequency to maintain stable magnetic flux.
• Advantages: Simple structure and strong versatility.
• Disadvantages: Insufficient torque at low speeds and slow dynamic response.
• Applications: Scenarios with low requirements for dynamic performance, such as fans and water pumps.

2.Vector Control

• It is an advanced control mode. Through complex mathematical calculations (coordinate transformation), it can independently and precisely control the magnetic field current (excitation component) and torque current (torque component) of an AC motor, just like controlling a DC motor.
• Advantages: Large torque at low speeds, extremely fast dynamic response (quick start-stop and speed regulation), and high control precision.
• Applications: Scenarios requiring high precision and high dynamic performance, such as cranes, elevators, machine tools, and textile machinery.